Clean up hashes and related types ahead of `bitcoin_hashes` 1.0 #716

Job	Run time
Docs - nightly toolchain	1m 32s
Docs - stable toolchain	1m 29s
Test - nightly toolchain (recent)	6m 6s
Test - msrv toolchain (recent)	1m 50s
Cross test	1m 51s
Lint - nightly toolchain	1m 51s
Bench - nightly toolchain	1m 35s
Test 32-bit version	59s
Test - stable toolchain (recent)	6m 25s
Check WASM	41s
	24m 19s

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